1. Field of the Invention
The present invention relates to data transmission in a wireless communication system, especially to a method for variable sub-carrier mapping and device using the same.
2. Description of the Related Art
Compared with the 3G system at present, the evolved next generation mobile communication system offers shorter transmission delay (including the time on access, air-interface transmission, network process and network transmission), higher user uplink/downlink data transmission rate, higher spectrum utilization factor, larger system coverage, and in the meantime, it reduces the network operator's construction cost and maintenance cost as much as possible. To meet the demand mentioned above, technique schemes such as AMC, HARQ, OFDM(A) (including Localized OFDM and Distributed OFDM) and SC-FDMA are being evaluated at present and may be adopted in the next generation mobile communication system. The transmission mechanism of Hybrid Automatic Retransmission Request (HARQ) is adopted in the uplink/downlink data service. With the technique of data retransmission, time diversity and combining gain can be obtained so that system's throughput rate can be effective improved.
OFDM (Orthogonal Frequency Division Multiplexing) is a special multi-carrier modulation/multiplexing technique. The block diagram of its transmitter/receiver is shown in FIG. 1. A single user's information stream is converted into multiple low-rate code streams from a serial to parallel which are simultaneously transmitted via a group of sub-carriers whose spectrums are superposed but orthogonal. The advantages of OFDM technique are as follows:
1) Better performance in anti-frequency selective fading and anti-narrow band interference. In a single-carrier system, a single fading or interference may cause the entire link out of work. But in a multi-carrier system, only a few carriers will be affected. In an OFDM system, user information stream is converted into multiple low-rate information streams from serial to parallel which are simultaneously transmitted via a group of sub-carriers. Each sub-carrier's signaling time is times longer than that in the single-carrier system with the same rate. This better improves OFDM's performance in anti-narrow-band interference and the channel's anti-fast fading. Meanwhile, with the sub-carrier's combining coding, the frequency diversity effect is gained for sub-channels so that the performance in anti-narrow-band interference and channel's anti-fast fading is improved.
2) Higher frequency utilization factor. OFDM adopts superposed but orthogonal sub-carriers as sub-channels, differing from the traditional system that applies guard bands in sub-channel's dividing, so that the frequency utilization factor is improved.
3) Suitable for data transmission in high rate. With the adaptive modulation mechanism, an OFDM system can apply different modulation schemes in different sub-carrier's modulation according to the channel conditions and the noise backgrounds. When the channel is in good condition, modulation scheme with high efficiency is applied. In addition, when the channel is in poor condition, the modulation scheme with powerful performance in anti-interference is applied. Furthermore, with the application of loadable algorithm, more data can be transmitted concentratively in high data rate via the channel with good conditions in the OFDM system. Thus, the technique of OFDM is very suitable for data transmission in high rate.
4) Better performance in anti-Inter-Symbol Interference (ISI). Besides the noise interference, ISI is the main interference in a digital communication system. Since OFDM adopts the cyclic prefix, it has better performance in anti-ISI. The capability in anti-frequency selective fading and anti-narrow-band interference is improved in OFDM. In a single-carrier system, one fading or interference can cause the entire link out of work, but in a multi-carrier system, only a few carriers would be affected.
5) In the technique of OFDM, the modulation/demodulation can be realized through the base-band IFFT/FFT, which bears available fast calculation method and can be conveniently implemented in a DSP chip and hardware structure.
However, OFDM has following disadvantages:
Sensitive to frequency deviation and phase noise so that attenuation is easily caused to the system; and
comparatively higher Peak-to-Average Power Ratio (PAPR), which results in that the RF amplifier's power efficiency is poor.
Since the PAPR is high in a multi-carrier system, and considering factors such as the mobile set's transmission power, its size, its stand-by time, and the cell's coverage and so on, the technique of Single-Carrier Frequency Division Multiple Access (SC-FDMA) may be possibly adopted in the uplink of the next generation mobile communication system. Still multiple sub-carriers are adopted to transmit signals in a SC-FDMA system. But there's some difference between the SC-FDMA system and the multi-carrier system: in the multi-carrier system, each sub-carrier transmits a single modulated symbol, and in the SC-FDMA system, each sub-carrier transmits the whole modulated symbols information. The SC-FDMA signal can be generated in time domain or frequency domain approaches. The structure of the transmitter/receiver (the frequency domain implementation approach) is shown in FIG. 2.
HARQ (Hybrid Automatic Retransmission Request) is a link adaptive technique which is the marriage of Forward Error Correction (FEC) Coding and Automatic Retransmission Request (ARQ). With the application of FEC, the transmission reliability is improved. But in the case of better channel conditions, system's throughput is on the contrary reduced because of excessive error correction bits. In the case of not high error bit rate, ideal throughput can be obtained with ARQ. But the ARQ will bring about extra retransmission delay, so that it is considered to combine FEC with ARQ to generate the Hybrid ARQ. Each transmitted data packet contains check bits for error correction and error detection. If the number of error bits in the received data packet is within the range that can be corrected, any error will be corrected automatically; but if severe error (which is beyond the range that FEC can correct) comes across, retransmission is requested to the transmitter. HARQ is able to adaptively adjust with the change of the channel, i.e., it can elaborately regulate the data rate according to the channel conditions.
In order to make full use of the system resources and reduce the overhead in signalling and buffer, the N-channel Stop-and-Wait (N-SAW) HARQ transmission mechanism is applied in the system, as is shown in FIG. 3 in principle. With the N-SAW HARQ, data packets of N HARQ processes can be transmitted via a single channel. When the forward link is adopted to transmit the data packet of some HARQ process, the backward link is applied to transmit the response information of other HARQ processes. With the N-SAW HARQ, data transmission can be operated continuously via the forward link. The system resources are fully utilized. But in this case, it is necessary for the receiver to be able to buffer the information of N data packets.
N-SAW HARQ can be divided into two cases:
1) N-SAW synchronous HARQ: the HARQ process is initiated only at the specific moment.t=m+k×N (k=1, 2, . . . , nmax)  (1)
Where: t denotes the TTI of retransmission; m denotes the TTI of the original transmission; nmax denotes the maximum number of retransmission of HARQ; N denotes the number of HARQ processes.
2) N-SAW asynchronous HARQ: the HARQ process can be initiated at any moment after the response information to the previous data packet is received.t≧m+N  (2)
Where: t denotes the TTI of retransmission; m denotes the TTI for the transmission of the previous data packet; N denotes the number of HARQ processes.
To meet the demand of delay, shorter Transmission Time Interval (referred to as TTI) will be adopted in the next generation mobile communication system. Three possible TTI's lengths can be 0.5 ms, 0.625 ms and 0.667 ms. The TTI is used as the base time interval in N-SAW HARQ. In the N-SAW synchronous HARQ, the retransmission interval for the same data packet is N·TTI, and in N-SAW asynchronous HARQ, the retransmission interval for the same data packet is k·N·TTI (i<k<nmax) where: nmax denotes the maximum number of retransmission in the HARQ process. In the case that N TTI is greater than channel's coherence time, the channel fading experienced by the retransmitted data packet in a HARQ process is different from that experienced by the data packets transmitted N TTI before in the HARQ process. And in the case that N TTI is less than channel's coherence time, the channel fading experienced by the retransmitted data packet in a HARQ process is similar to that experienced by the data packets transmitted N TTI before in the HARQ process. Although we can increase N to make N·TTI greater than channel's coherence time, it is not fit for the next generation mobile communication system, for the next generation mobile communication system adopts shorter TTI. Because the increase of N results in the increase of receive buffer (N HARQ processes corresponds to N buffers for soft-combination). In the meantime, the increase of N results in the increase of average delay (average delay=HARQ average retransmission times×N×TTI).
In a word, there is a problem going along with the application of the HARQ transmission mechanism in the next generation mobile communication system as follows:
Consecutive data packets transmitted in the same HARQ process experience similar channel fading. The sub-carrier adopted to transmit data packets by HARQ experiences deep fading, and so does to this sub-carrier during the process of retransmission.